1. Field of the Invention
This invention relates to graphic presentation generating circuits including display character generators and plotter control methods and apparatus.
2. Description of the Prior Art
Systems for generating circles and elipses on a cathode ray tube are generally known in the prior art. Examples include systems such as disclosed in U.S. Pat. No. 3,984,664 wherein the output of X and Y coordinate counters are provided to a read only memory to generate the square of input values which are then added and compared to control the video input to a TV type raster display. Due to the time required to repetitively calculate x.sup.2 +y.sup.2 values by table lockup or otherwise, methods obviating calculation of squares have been developed. Examples are Pitteway, M. L. V. Algorithm for drawing ellipses or hyperbolae with a digital plotter Comptr. J. 10, 3 (Nov. 1967), 282-289; J. E. Bresenham, A linear algorithm for incremental digital display of circular arcs Communications of the ACM 20, 2 (Feb 1977), 100-106; Z. Kulpa, On the properties of discrete circles, rings, and disks, Computer Graphics and Image Processing, Volume 10, Number 4, (August 1979) p. 348-365; and M. Doros, Algorithm for generation of discrete circles, rings, and disks, ibid p. 366-371. The Pitteway reference teaches the concept of drawing by octant but does not teach efficient initiation or termination of circular arcs of arbitrary length (note error criteria is also different). Although the Bresenham ACM reference has a simpler reinitialization, the repetitive stepping loop is longer than might be desired. B. W. Jordan, W. J. Lennon and B. C. Holm, An improved algorithm for the generation of nonparametric curves, IEEE Trans Comp C-22, 12 (Dec. 1973), 1052-1060, gives three possibilities for approaches to termination. However this reference looks at each point with the full test rather than doing preliminary calculations to localize termination testing.